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Nightmare Fuel: Dishonored
The Heart. Not only is it a talking undead human heart kept alive by magic and clockwork, it also knows every secret, no matter how dark or terrifying about every place and every person it encounters, it knows what it is, but wishes it was dead, and to top it all off it's the remains of the Empress!
And each time you point it at rune or bone charm it starts beating...
Some of the deaths performed are rather gruesome. For example:
You can summon rats to devour your enemies alive and whole. You can even watch as the clothes and skin are stripped off to reveal the muscles and bones, which doubles neatly as Nausea Fuel.
Even worse, if you see a swarm of rats running around a level, better be careful - they're hungry and if there's no corpse around to distract them, they'll eat you.
You can possess people into either jumping off balconies to their deaths or walking into the path of their own bullet.
You can fill a room with scalding steam, burning your target and anyone with them.
The masks worn in Lady Boyle's party. Some are enlarged insect heads complete with antennae and compound eyes. One of them is an uncanny baby, with the rest of the "baby's" body forming a hat.
Even Creepier is the mask for an important character in that mission. The mask looks like a crude bag with hairs sticking out of it. He's important because he provides the nonlethal option for that mission. The nonlethal option itself being several layers of creepy. He is a Stalker with a Crush who offers to take Lady Boyle away to his own secret location where he will love her for ever and ever and ever and she will never be heard from again. Though there is less of a sting to it when you are informed she was in on the plan to kill the Empress.
While most of "The Hand That Feeds" is pretty damned disturbing, it's the end where the boy is lying on his back in a growing pool of blood, while blood runs freely from his eyes that is pure Nightmare Fuel.
And then his body turns into the in-universe plague warning symbol...
The Heart given by The Outsider that leads you to bone charms and whispers "secrets" as it beats in your hand.
Dialogue overheard from Piero and by the heart itself implies that the heart is the Empress's, and that her soul is trapped inside.
The developers have confirmed that it is her, pushing this to And I Must Scream territory.
The setting in general. You've got a steampunk/dieselpunk, dystopian city run by a corrupt regime while swarms of rats spread a plague that causes people to turn into coughing, blood crying psychopaths nobody can find an inexpensive cure for. Not to mention these victims are quarantined by guards authorized to use lethal force and electrical walls and pylons that turn anyone who gets too close to them into ash. Plus, don't forget Dunwall's class based system, as the poor are left to rot with this plague while the rich ignore the problem and throw lavish parties.
And don't forget the healthy dose of Lovecraftian Horror tossed in for good measure.
Also, if you get the plague, aside from being doomed to die a slow, painful death, one of two things will happen to you: You'll be reported to the city watch, who will quarantine you in the worst parts of the city where you'll live out the rest of your days surrounded by weepers, or you'll be hidden from the watch, and your loved ones will likely either get sick, or get arrested for not reporting a victim.
There's also the fact that Sokolov is experimenting on living plague victims.
No, it is far worse than that. While some of Sokolov's experiments may be performed on plague victims, he specifically chooses healthy citizens to test out his cures for the rat plague, experiments which involve him first infecting them with the disease itself. During the mission to capture him, Corvo (and therefore the player) is treated to the sight of several civilians being held in electric cages waiting to be experimented upon. Add to the fact that the guards pretty much dismiss these poor people as "pigs" to be slaughtered except for one obviously disturbed man who is horrified at the idea of doing this to innocent people.
Weepers. These are not zombies, but being a zombie would be a mercy compared to what they have become: they shamble around, continuously vomiting, their vomit being highly caustic (it injures you if they vomit on you), and their bodies have become home to large numbers of flies. Maggots are literally devouring their flesh while they are still alive. When they clutch you when they attack, they aren't trying to hurt you, they're begging for help, or at least for you to put them out of their misery.
Some of the non-lethal version for eliminating the targets. Lady Boyle's fate was already bad enough, but at least she's more or less unharmed, for the moment. Campbell, however, you effectively turn into an unperson, and as a result he catches the plague and dies in agony, while you leave the Pendleton twins to be abducted, have their tongues cut out and be sent off to be worked to death.Pay Evil unto Evil and all that, but still, sheesh.
Go ahead an empathize with your targets to realize just how pants-shittingly terrifying it is to have a visit from Corvo. For example, when you go to abduct Sokolov, you can make the choice to talk to him instead of just straight-up knocking him out. Just stand there for a minute, silent, watching, and eventually he'll freak out and run past you, screaming for help...only to find that all of the dozen-plus guards are dead. The entire map is empty. Just him, Alone With The Assassin.
People really don't give guards enough credit, considering what they're up against. Go and have another good look at the game's box art (or the main page's picture). We're comforted by knowing that's our PC, but that mask is monstrous. Add to that the fact most, if not all armed opposition are aware it's being worn by an in-universe Memetic Badass with allegedly no anti-murder scruples. Now imagine that crawling out of a shadow you knew nobody could possibly have been in (without outsider help) and dropping whisper-quiet to the ground in front of you. It must take no small amount of courage not to run screaming.
The in-game song, "Drunken Whaler", is much darker than the "Drunken Sailor" song it's based on. The original has some moderately reasonable suggestions. "Drunken Whaler"... Not so much - all lethal, and a good sign of how bad off this city is.
Feed 'im to the hungry rats for dinner...
In the "Burglar" mission of Dunwall City Trials, you can find a hidden nook behind a wall if you pull a switch. Enter the room, grab what's in there, turn around to leave... and suddenly the exit is instead a pitch-black void filled with busts of Hiram Burrows hanging in mid-air, with a "shrine" to him in the middle that looks like a mockery of an Outsider Shrine. And there's no apparent other way out.
Same mission: There's a safe with no combination that you need to open for a few ingots. On and around said safe are several busts of Burrows. Walk into the corner to hide from a guard, turn around... And the busts have turned to stare at you.
Every time you finish Kill Cascade, you're drop-assassinating Empress Jessamine right in front of Emily.
Corvo build of Bend Time 2 combined with Shadow Kill can be one for his targets when you think about it. A guard leaves a room to grab some wine. When he gets back, all three of his friends have vanished. He only sees what looks like black ash falling to the ground. Said guard is joined by two more friends, who witness what looks like death incarnate blinking into existence, making a complicated gesture and vanishing again. Poor guards barely have enough time to know death is coming and there is absolutely nothing they can do about it.
The Rothwild Slaughterhouse. Lots of whale gore everywhere, a live whale undergoing a slow, painful and gruesome death and singing a creepy song, and the butchers, who really enjoy hacking Daud to pieces with their saws make for a rather disturbing mission.
In the Rothwild Slaughterhouse you find a note listing the number of accidents, injuries and deaths that have occurred there. Three numbers. Bundy is using the number of maimed workers as a safe code.
You see another example of Bundy's frightening sociopathy in an audio log where he taunts the tortured whale hung up on chains and facing his office. The sad songs of the dying whales haunt the dreams of his workers, but for him they are amusing.
The Geezer's whale-oil life support machine. Especially if you read Trimble's notes.
When Pretty Emily woke one day,
She saw a world a different way.
Her eyes now looked with a stranger's guile,
Her dainty mouth smiled a stranger's smile,
Her hands now worked the stranger's wrath,
Her feet now walked a stranger's path.
Emily fed, another grew stronger.
The stranger's cravings drove her onward,
And no one who looked on Emily's face
Ever guessed who ruled in Emily's place.
The non-lethal solution to the final mission of The Brigmore Witches: Daud swaps the painting of Emily with a painting of The Void, trapping Delilah there forever.
Corrupted Bone Charms bestow power like regular Bone Charms, however they come with a heavy price. One account mentions one that made a tooth turn black and fall out whenever used. The author passes it on to someone else who ends up with a mouth full of blood and gums and the author wonders what parts of inside him have turned black.
In Brigmore Manor,Daud can find the sole survivor of an Overseer team sent to scout out and monitor the compound for heretical activity. He's curled up in the corner of a side room, raving madly, and surrounded by piles of gnawed on flesh. Delilah's coven spotted and captured both men, and attempted to torture information on the Abbey's knowledge of the witches out of them. When neither talked, they killed one Overseer and force fed the other his flesh until he nearly burst.
The Void, with its fragments of buildings and scenery floating in nothingness, displaying characters from the "real world" frozen in time, some in the act of dying. The glowy blue apparition of the Outsider doesn't help matters.
The Outsider looks especially demonic, if not outright Satanic when he appears before you after finding the Rune in Granny Rags' shrine to him in the Old Dunwall Sewers towards the end of the Flooded District level; He's bathed in an eerie red light from the shrine's candles, but because he's hovering in the air and looking down at Corvo, his eyes are heavily shadowed, as are two points on his forehead, giving him the impression of horns. It's probably the single-most unnerving of his appearances, which seems fitting considering the Flooded District ends up pitting you against two other people who have received his Mark; Daud (the Empress' assassin, no less) and Granny Rags.